ZSM-5 Containing aluminum-free shells on its surface

ABSTRACT

Aluminosilicate zeolites are prepared containing an outer aluminum-free shell. The outer shell is essentially SiO 2  that has crystallized on the zeolite surface in the ZSM-5 type configuration, leading to a more selective catalyst.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application U.S. Ser. No. 868,147,filed Jan. 9, 1978, now U.S. Pat. No. 4,148,713, which in turn was acontinuation-in-part of application U.S. Ser. No. 726,353, filed Sept.24, 1976, now U.S. Pat. No. 4,088,605.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to crystalline aluminosilicate zeolites and tothe synthesis thereof. It more particularly relates to the synthesis ofa zeolite containing an outer shell free from aluminum. The inventionfurther relates to the product of such synthesis.

2. Discussion of the Prior Art

Certain of the zeolites disclosed herein and their synthesis are wellknown. Examples are ZSM-5 and ZSM-11. These zeolites are fully describedin U.S. Pat. Nos. 3,702,886 and 3,709,979. They are known to havecatalytic capabilities for various conversion reactions. Because oftheir ordered, porous structure, creating interconnected cavities, theyare selective toward certain molecules. That is to say, the pores acceptfor adsorption molecules of certain dimensions while rejecting those oflarger dimensions. However, no known art discloses or suggestsincreasing selectivity by essentially inactivating the surface of thecatalyst with an isocrystalline layer of aluminum-free zeolite.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a crystallinealuminosilicate zeolite having an aluminum-free outer shell ofcrystalline SiO₂, the zeolite being made by a two-stage methodcomprising:

(1) initiating crystallization in a crystallization medium to producethe zeolite and then

(2) altering the crystallization medium to eliminate the aluminumtherein, wherein said outer shell of SiO₂ has the same crystal structureas said zeolite. In many cases it will also be desirable to increase thehydroxide content and/or to reduce the organic ion, i.e., the templateion, to SiO₂ ratio.

In a broader aspect, it is apparent that the invention affords a newcomposition not limited by any process steps. Thus, there is alsoprovided a crystalline zeolite having a core comprising athree-dimensional network of SiO₄ and AlO₄ tetrahedra cross-linked bythe sharing of oxygen atoms and an outer shell having the same crystalstructure, but consisting essentially of silica. Stated another way, theinvention provides a crystalline aluminosilicate zeolite having analuminum-free outer shell of SiO₂, said outer shell having the samecrystal structure as said zeolite.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The zeolite catalysts useful herein are ZSM-5 type zeolites and aremembers of a class of zeolites exhibiting some unusual properties. Theyare useful in cracking and hydrocracking and are outstandingly useful inother petroleum refining processes, indicating again the uniquecatalytic characteristics of this family of zeolites. The latterprocesses include isomerization of n-paraffins and naphthenes,polymerization of compounds containing an olefinic or acetylenic carbonto carbon linkage such as isobutylene and butene-1, reforming,alkylation, isomerization of polyalkyl substituted aromatics, e.g.,ortho xylene, aromatics alkylation, such as reaction of benzene withethylene, disproportionation of aromatics such as toluene to provide amixture of benzene, xylenes and higher methylbenzenes, as well asconversion of polar compounds such as methanol to hydrocarbon products.They have exceptional high selectivity and under the conditions ofhydrocarbon conversion provide a high percentage of desire productsrelative to total products compared with known zeolite hydrocarbonconversion.

Although they have unusually low alumina contents, i.e. high silica toalumina ratios, they are very active even when the silica to aluminaratio exceeds 30. The activity is surprising since catalytic activity isgenerally attributed to framework aluminum atoms and cations associatedwith these aluminum atoms. These catalysts retain their crystallinityfor long periods in spite of the presence of steam at high temperaturewhich induces irreversible collapse of the framework of other zeolites,e.g. of the X and A type. Furthermore, carbonaceous deposits, whenformed, may be removed by burning at higher than usual temperatures torestore activity. In many environments the zeolites of this classexhibit very low coke forming capability, conducive to very long timeson stream between burning regenerations.

An important characteristic of the crystal structure of this class ofzeolites is that it provides constrained access to, and egress from, theintracrystalline free space by virtue of having a pore dimension greaterthan about 5 Angstroms and pore windows of about a size such as would beprovided by 10-membered rings of oxygen atoms. It is to be understood,of course, that these rings are those formed by the regular dispositionof the tetrahedra making up the anionic framework of the crystallinealuminosilicate, the oxygen atoms themselves being bonded to the siliconor aluminum atoms at the centers of the tetrahedra. Briefly, thepreferred type catalysts useful in this invention possess, incombination: a silica to alumina ratio of at least about 12; and astructure providing a constrained access to the crystalline free space.

The silica to alumina ratio referred to may be determined byconventional analysis. This ratio is meant to represent, as closely aspossible, the ratio in the rigid anionic framework of the zeolitecrystal and to exclude aluminum in the binder or in cationic or otherform within the channels. Although catalysts with a silica to aluminaratio of at least 12 are useful, it is preferred to use catalysts havinghigher ratios of at least about 30. Such catalysts, after activation,acquire an intracrystalline sorption capacity for normal hexane which isgreater than that for water, i.e. they exhibit "hydrophobic" properties.It is believed that this hydrophobic character is advantageous in thepresent invention.

The type zeolites useful in this invention freely sorb normal hexane andhave a pore dimension greater than about 5 Angstroms. In addition, thestructure must provide constrained access to larger molecules. It issometimes possible to judge from a known crystal structure whether suchconstrained access exists. For example, if the only pore windows in acrystal are formed by 8-membered rings of oxygen atoms, then access bymolecules of larger cross-section than normal hexane is excluded and thezeolite is not of the desired type. Windows of 10-membered rings arepreferred, although, in some instances, excessive puckering or poreblockage may render these catalysts ineffective. Twelve-membered ringsdo not generally appear to offer sufficient constraint to produce theadvantageous conversions, although puckered structures exist such as TMAoffretite which is a known effective zeolite. Also, structures can beconceived, due to pore blockage or other cause, that may be operative.

Rather than attempt to judge from crystal structure whether or not acatalyst possesses the necessary constrained access, a simpledetermination of the "constraint index" may be made by passingcontinuously a mixture of an equal weight of normal hexane and3-methylpentane over a small sample, approximately 1 gram or less, ofcatalyst at atmospheric pressure according to the following procedure. Asample of the catalyst, in the form of pellets or extrudate, is crushedto a particle size about that of coarse sand and mounted in a glasstube. Prior to testing, the catalyst is treated with a stream of air at1000° F. for at least 15 minutes. The catalyst is then flushed withhelium and the temperature adjusted between 550° F. and 950° F. to givean overall conversion between 10% and 60%. The mixture of hydrocarbonsis passed at 1 liquid hourly space velocity (i.e., 1 volume of liquidhydrocarbon per volume of catalyst per hour) over the catalyst with ahelium dilution to give a helium to total hydrocarbon mole ratio of 4:1.After 20 minutes on stream, a sample of the effluent is taken andanalyzed, most conveniently by gas chromatography, to determine thefraction remaining unchanged for each of the two hydrocarbons.

The "constraint index" is calculated as follows:

    Constraint Index=log.sub.10 (fraction of n-hexane remaining)/log.sub.10 (fraction of 3-methylpentane remaining)

The constraint index approximates the ratio of the cracking rateconstants for the two hydrocarbons. Catalysts suitable for the presentinvention are those having a constraint index in the approximate rangeof 1 to 12. Constraint Index (CI) values for some typical catalysts are:

    ______________________________________                                        CAS                      C.I.                                                 ______________________________________                                        ZSM-5                    8.3                                                  ZSM-11                   8.7                                                  ZSM-12                   2                                                    ZSM-21                   4.5                                                  ZSM-35                   4.5                                                  TMA Offretite            3.7                                                  Beta                     0.6                                                  ZSM-4                    0.5                                                  H-Zeolon                 0.5                                                  REY                      0.4                                                  Amorphous Silica-Alumina 0.6                                                  Erionite                 38                                                   ______________________________________                                    

It is to be realized that the above constraint index values typicallycharacterize the specified zeolites but that such are the cumulativeresult of several variables used in determination and calculationthereof. Thus, for a given zeolite depending on the temperature employedwithin the aforenoted range of 550° F. to 950° F., with accompanyingconversion between 10% and 60%, the constraint index may vary within theindicated approximate range of 1 to 12. Likewise, other variables suchas the crystal size of the zeolite, the presence of possibly occludedcontaminants and binders intimately combined with the zeolite may affectthe constraint index. It will accordingly be understood by those skilledin the art that the constraint index, as utilized herein, whileaffording a highly useful means for characterizing the zeolites ofinterest is approximate, taking into consideration the manner of itsdetermination, with the probability, in some instances, of compoundsvariable extremes. However, in all instances, at a temperature withinthe above-specified range of 550° F. to 950° F., the constraint indexwill have a value for any given zeolite of interest herein within theapproximate range of 1 to 12.

The class of zeolites defined herein is defined by ZSM-5, ZSM-11,ZSM-12, ZSM-21 and ZSM-35. U.S. Pat. No. 3,702,886, as mentioned above,describes and claims ZSM-5. The patent is incorporated herein byreference.

ZSM-5 type zeolite compositions have the characteristic X-raydiffraction pattern set forth in Table 1, hereinbelow. ZSM-5 itself canalso be identified, in terms of mole ratios of oxides as follows:##STR1## wherein M is a cation, n is the valence of said cation, W isaluminum, Y is silicon, and z is from 0 to 40. In a preferredsynthesized from, the zeolite has a formula, in terms of mole ratios ofoxides, as follows: ##STR2## and M is selected from the group consistingof a mixture of alkali metal cations, especially sodium, and organicions, such as tetraalkylammonium cations, the alkyl groups of whichpreferably contain 2-5 carbon atoms.

The original cations can be replaced in accordance with technqueswell-known in the art, at least in part, by ion exchange with othercations. Preferred replacing cations include tetraalkylammonium cations,metal ions, ammonium ions, hydrogen ions, and mixtures of the same.Particularly preferred cations are those which render the zeolitecatalytically active, especially for hydrocarbon conversion. Theseinclude hydrogen, rare earth metals, aluminum, metals of Groups II andVIII of the Periodic Table.

In a preferred embodiment of ZSM-5, W is aluminum, Y is silicon and thesilica/alumina mole ratio is at least 10 and ranges up to about 300.

ZSM-5 type zeolites have an exceptionally high degree of thermalstability, thereby rendering them particularly effective for use inprocesses involving elevated temperatures. In this connection, ZSM-5type zeolites appear to be one of the most stable families of zeolitesknown to date.

ZSM-5 zeolite possesses a definite distinguishing crystalline structurewhose X-ray diffraction pattern shows the following significant lines:

                  TABLE 1                                                         ______________________________________                                        Interplanar                                                                   Spacing d(A)         Relative Intensity                                       ______________________________________                                        11.1       ±    0.2       s.                                               10.0       ±    0.2       s.                                               7.4        ±    0.15      w.                                               7.1        ±    0.15      w.                                               6.3        ±    0.1       w.                                               6.04                                                                                      ±   0.1       w.                                               5.97                                                                          5.56       ±    0.1       w.                                               5.01       ±    0.1       w.                                               4.60       ±    0.08      w.                                               4.25       ±    0.08      w.                                               3.85       ±    0.07      v.s.                                             3.71       ±    0.05      s.                                               3.04       ±    0.03      w.                                               2.99       ±    0.02      w.                                               2.94       ±    0.02      w.                                               ______________________________________                                    

These values were determined by standard techniques. The radiation wasthe K-alpha doublet of copper, and a scintillation counter spectrometerwith a strip chart pen recorder was used. The peak heights, I, and thepositions as a function of 2 times theta, where theta is the Bragg anglewere read from the spectrometer chart. From these, the relativeintensities, 100 I/I_(o) where I_(o) is the intensity of the strongestline or peak, and d (obs.), the interplanar spacing in A, correspondingto the recorded lines, were calculated. In Table 1 the relativeintensities are given in terms of the symbols s.=strong, m.=medium,m.s.=medium strong, m.w.=medium weak and v.s.=very strong. It should beunderstood that this X-ray diffraction pattern is characteristic of allthe species of ZSM-5 compositions. Ion exchange of the sodium ion withcations reveals substantially the same pattern with some minor shifts ininterplanar spacing and variation in relative intensity. Other minorvariations can occur depending on the silicon to aluminum ratio of theparticular sample and on whether it had been subjected to thermaltreatment.

Various cation exchanged forms of ZSM-5 have been prepared. X-ray powderdiffraction patterns of several of these forms of ZSM-5 are set forthfully in U.S. Pat. No. 3,702,886.

Zeolite ZSM-5 per se can be suitably prepared by preparing a solutioncontaining tetrapropyl ammonium hydroxide, sodium oxide, an oxide ofaluminum, an oxide of silica, and water and having a composition, interms of mole ratios of oxides, falling within the following ranges:

                  TABLE 2                                                         ______________________________________                                                                     Particularly                                                Broad   Preferred Preferred                                        ______________________________________                                         ##STR3##     0.02-10.0                                                                               0.05-0.8  0.2-0.75                                     ##STR4##     0.01-0.95                                                                               0.02-0.6  0.05-0.4                                     ##STR5##     10-1000   30-700    50-500                                       ##STR6##     5-2000    10-500    20-150                                      ______________________________________                                    

wherein R is propyl, W is aluminum and Y is silicon maintaining themixture until crystals of the zeolite are formed. It is noted that anexcess of tetrapropyl-ammonium hydroxide can be used which would raisethe value of OH--/YO₂ above the ranges set forth supra. The excesshydroxide, of course, does not participate in the reaction. Thereafter,the crystals are separated from the liquid and recovered. Typicalreaction conditions consist of heating the foregoing reaction mixture toa temperature of from about 80° C. to 200° C. for a period of time offrom about four hours to 180 days. A more preferred temperature range isfrom about 150° to 175° C. with the amount of time at a temperature insuch range being from about 4 hours to 8 days.

The digestion of the gel particles is carried out until crystals form.The solid product is separated from the reaction medium, as by coolingthe whole to room temperature, filtering, and water washing.

The foregoing product is dried, e.g., at 230° F., for from about 2 to 24hours. Of course, milder conditions may be employed if desired, e.g.,room temperature under vacuum.

The zeolites are obviously formed as aluminosilicates. The specificcomposition can be prepared utilizing materials which supply theappropriate oxide. Such compositions include, for an aluminosilicate,sodium aluminate, alumina, sodium silicate, silica hydrosol, silica gel,silicic acid, sodium hydroxide and tetrapropylammonium hydroxide. Itwill be understood that each oxide component utilized in the reactionmixture for preparing the zeolite can be supplied by one or more initialreactants and they can be mixed together in any order. For example,sodium oxide can be supplied by an aqueous solution of sodium hydroxide,or by an aqueous solution of sodium silicate-tetrapropylammonium cationcan be supplied by the bromide salt. The reaction mixture can beprepared either batchwise or continuously. Crystal size andcrystallization time of the ZSM-5 composition will vary with the natureof the reaction mixture employed.

ZSM-11 is described in U.S. Pat. No. 3,709,979, the entire contents ofwhich are incorporated herein by reference.

ZSM-12 is described in U.S. Pat. No. 3,832,449, the entire contents ofwhich are incorporated herein by reference.

ZSM-21 is described in U.S. application Ser. No. 528,060 (Mobil Docket8713), filed Nov. 29, 1974. This zeolite can be identified, in terms ofmole ratios of oxides and in the anhydrous state, as follows:

    (0.3-2.5)R.sub.2 O:(0-0.8)M.sub.2 O:Al.sub.2 O.sub.3 :>8 SiO.sub.2

wherein R is an organic nitrogen-containing cation derived from a2-(hydroxyalkyl) trialkylammonium compound and M is an alkali metalcation, and is characterized by a specified X-ray powder diffractionpattern.

In a preferred synthesized form, the zeolite has a formula, in terms ofmole ratios of oxides and in the anhydrous state, as follows:

    (0.4-2.5)R.sub.2 O:(0-0.6)M.sub.2 O:Al.sub.2 O.sub.3 :xSiO.sub.2

wherein R is an organic nitrogen-containing cation derived from a2-(hydroxyalkyl)trialkylammonium compound, wherein alkyl is methyl,ethyl or a combination thereof, M is an alkali metal, especially sodium,and x is from greater than 8 to about 50.

The synthetic ZSM-21 zeolite possesses a definite distinguishingcrystalline structure whose X-ray diffraction pattern showssubstantially the significant lines set forth in Table 3. It is observedthat this X-ray diffraction pattern (significant lines) is similar tothat of natural ferrierite with a notable exception being that naturalferrierite patterns exhibit a significant line at 11.33 A.

                  TABLE 3                                                         ______________________________________                                        Interplanar                                                                   Spacing d(A)       Relative Intensity                                         ______________________________________                                        9.8  ± 0.20     s.                                                         9.1 ± 0.19      m.                                                         8.0 ± 0.16      w.                                                         7.1 ± 0.14      m.                                                         6.7 ± 0.14      m.                                                         6.0  ± 0.12     w.                                                         4.37 ± 0.09     w.                                                         4.23 ± 0.09     w.                                                         4.01 ± 0.08      v.s.                                                      3.81 ± 0.08      v.s.                                                      3.69 ± 0.07     m.                                                         3.57 ± 0.07      v.s.                                                      3.51 ± 0.07      v.s.                                                      3.34 ± 0.07     m.                                                         3.17 ± 0.06     s.                                                         3.08 ± 0.06     m.                                                         3.00 ± 0.06     w.                                                         2.92 ± 0.06     m.                                                         2.73 ± 0.06     w.                                                         2.66 ± 0.05     w.                                                         2.60 ± 0.05     w.                                                         2.49 ± 0.05     w.                                                         ______________________________________                                    

A further characteristic of ZSM-21 is its sorptive capacity providingsaid zeolite to have increased capacity for 2-methylpentane (withrespect to n-hexane sorption by the ratio n-hexane/2-methyl-pentane)when compared with a hydrogen form of natural ferrierite resulting fromcalcination of an ammonium exchanged form. The characteristic sorptionratio n-hexane/2-methylpentane for ZSM-21 (after calcination at 600° C.)is less than 10, whereas that ratio for the natural ferrierite issubstantially greater than 10, for example, as high as 34 or higher.

Zeolite ZSM-21 can be suitably prepared by preparing a solutioncontaining sources of an alkali metal oxide, preferably sodium oxide, anorganic nitrogen-containing oxide, an oxide of aluminum, an oxide ofsilicon and water and having a composition, in terms of mole ratios ofoxides, falling within the following ranges:

    ______________________________________                                        R.sup.+        Broad        Preferred                                         ______________________________________                                        R.sup.+ + M.sup.+                                                                            0.2-1.0      0.3-0.9                                            ##STR7##       0.05-0.5     0.07-0.49                                         ##STR8##       41-500       100-250                                           ##STR9##       8.8-200      12-60                                            ______________________________________                                    

wherein R is an organic nitrogen-containing cation derived from a2-(hydroxyalkyl) trialkylammonium compound and M is an alkali metal ion,and maintaining the mixture until crystals of the zeolite are formed.(The quantity of OH-- is calculated only from the inorganic sources ofalkali without any organic base contribution). Thereafter, the crystalsare separated from the liquid and recovered. Typical reaction conditionsconsist of heating the foregoing reaction mixture to a temperature offrom about 90° C. to about 400° C. for a period of time of from about 6hours to about 100 days. A more preferred temperature range is fromabout 150° C. to about 400° C. with the amount of time at a temperaturein such range being from about 6 hours to about 80 days.

The digestion of the gel particles is carried out until crystals form.The solid product is separated from the reaction medium, as by coolingthe whole to room temperature, filtering and water washing. Thecrystalline product is thereafter dried, e.g., at 230° F. for from about8 to 24 hours.

ZSM-35 is more particularly described in U.S. application Serial No.528,061 (Mobil Docket 8714), filed November 29, 1974. This zeolite canbe identified, in terms of mole ratios of oxides and in the anhydrousstate, as follows:

    (0.3-2.5)R.sub.2 O:(0-0.8)M.sub.2 O:Al.sub.2 O.sub.3 :>8 SiO.sub.2

wherein R is an organic nitrogen-containing cation derived frometheylenediamine or pyrrolidine and M is an alkali metal cation, and ischaracterized by a specified X-ray powder diffraction pattern.

In a preferred synthesized form, the zeolite has a formula, in terms ofmole ratios of oxides and in the anhydrous state, as follows:

    (0.4-2.5)R.sub.2 O:(0-0.6)M.sub.2 O:Al.sub.2 O.sub.3 :xSiO.sub.2

wherein R is an organic nitrogen-containing cation derived fromethylenediamine or pyrrolidine, M is an alkali metal, especially sodium,and x is from greater than 8 to about 50.

The synthetic ZSM-35 zeolite possesses a definite distinguishingcrystalline structure whose X-ray diffraction pattern showssubstantially the significant lines set forth in Table 4. It is observedthat this X-ray diffraction pattern (with respect to significant lines)is similar to that of natural ferrierite with a notable exception beingthat natural ferrierite patterns exhibit a significant line at 11.33 A.Close examination of some individual samples of ZSM-35 may show a veryweak line at 11.3-11.5 A. This very weak line, however, is determinednot to be a significant line for ZSM-35.

                  TABLE 4                                                         ______________________________________                                        Interplanar                                                                   Spacing d(A)       Relative Intensity                                         ______________________________________                                        9.6  ± 0.20     v.s. - v.v.s.                                              7.10 ± 0.15     m.                                                         6.98 ± 0.14     m.                                                         6.64 ± 0.14     m.                                                         5.78 ± 0.12     w.                                                         5.68 ± 0.12     w.                                                         4.97 ± 0.10     w.                                                         4.58 ± 0.09     w.                                                         3.99 ± 0.08     s.                                                         3.94 ± 0.08      m.s.                                                      3.85 ± 0.08     m.                                                         3.78 ± 0.08     s.                                                         3.74 ± 0.08     w.                                                         3.66 ± 0.07     m.                                                         3.54 ± 0.07      v.s.                                                      3.48 ± 0.07      v.s.                                                      3.39 ± 0.07     w.                                                         3.32 ± 0.07      w.m.                                                      3.14 ± 0.06      w.m.                                                      2.90 ± 0.06     w.                                                         2.85 ± 0.06     w.                                                         2.71 ± 0.05     w.                                                         2.65 ± 0.05     w.                                                         2.62 ± 0.05     w.                                                         2.58 ± 0.05     w.                                                         2.54 ± 0.05     w.                                                         2.48 ± 0.05     w.                                                         ______________________________________                                    

A further characteristic of ZSM-35 is its sorptive capacity proving saidzeolite to have increased capacity for 2-methylpentane (with respect ton-hexane sorption by the ratio n-hexane/2-methylpentane) when comparedwith a hydrogen form of natural ferrierite resulting from calcination ofan ammonium exchanged form. The characteristic sorption ration-hexane/2-methylpentane for ZSM-35 (after calcination at 600° C.) isless than 10, whereas that ratio for the natural ferrierite issubstantially greater than 10, for example, as high as 34 or higher.

Zeolite ZSM-35 can be suitably prepared by preparing a solutioncontaining sources of an alkali metal oxide, preferably sodium oxide, anorganic nitrogen-containing oxide, an oxide of aluminum, an oxide ofsilicon and water and having a composition, in terms of mole ratios ofoxides, falling within the following ranges:

    ______________________________________                                        R.sup.+        Broad        Preferred                                         ______________________________________                                        R.sup.+ + M.sup.+                                                                            0.2-1.0      0.3-0.9                                            ##STR10##      0.05-0.5     0.07-0.49                                         ##STR11##      41-500       100-250                                           ##STR12##      8.8-200      12-60                                            ______________________________________                                    

wherein R is an organic nitrogen-containing cation derived frompyrrolidine or ethylenediamine and M is an alkali metal ion, andmaintaining the mixture until crystals of the zeolite are formed. (Thequantity of OH-- is calculated only from the inorganic sources of alkaliwithout any organic base contribution). Thereafter, the crystals areseparated from the liquid and recovered. Typical reaction conditionsconsist of heating the foregoing reaction mixture to a temperature offrom about 90° C. to about 400° C. for a period of time of from about 6hours to about 100 days. A more preferred temperature range is fromabout 150° C. to about 400° C. with the amount of time at a temperaturein such range being from about 6 hours to about 80 days.

The digestion of the gel particles is carried out until crystals form.The solid product is separated from the reaction medium, as by coolingthe whole to room temperature, filtering and water washing. Thecrystalline product is dried, e.g. at 230° F., for from about 8 to 24hours.

The specific zeolites described, when prepared in the presence oforganic cations, are catalytically inactive, possibly because theintracrystalline free space is occupied by organic cations from theforming solution. They may, however, be activated by heating in an inertatmosphere at 1000° F. for one hour, followed by base exchange withammonium salts and followed by a further calcination at 1000° F. in air.

The zeolites can be used either in the alkali metal form, e.g., thesodium form, the ammonium form, the hydrogen form, or another univalentor multivalent cationic form. Preferably, one or the other of the lasttwo forms is employed. They can also be used in intimate combinationwith a hydrogenating component such as tungsten, vanadium, molybdenum,rhenium, nickel, cobalt, chromium, manganese, or a noble metal such asplatinum or palladium where a hydrogenation-dehydrogenation function isto be performed. Such component can be exchanged into the composition,impregnated therein or physically intimately admixed therewith. Suchcomponent can be impregnated in or on to the present catalyst such as,for example, by in the case of platinum, treating the zeolite with aplatinum metal-containing ion. Thus, suitable platinum compounds includechloroplatinic acid, platinous chloride and various compounds containingthe platinum amine complex.

The compounds of the useful platinum or other metals can be divided intocompounds in which the metal is present in the cation of the compoundand compounds in which it is present in the anion of the compound. Bothtypes which contain the metal in the ionic state can be used. A solutionin which platinum metals are in the form of a cation or cationiccomplex, e.g., Pt(NH₃)₆ Cl₄ is particularly useful. For some hydrocarbonconversion processes, this noble metal form of the catalyst isunnecessary such as in low temperature, liquid phase ortho xyleneisomerization.

The catalyst, when employed either as an adsorbent or as a catalyst inone of the aforementioned processes, should be dehydrated at leastpartially. This can be done by heating to a temperature in the range of200° to 600° C. in an atmosphere such as air, nitrogen, etc., and atatmospheric or subatmospheric pressures for between 1 and 48 hours.Dehydration can also be performed at lower temperatures merely byplacing the catalyst in a vacuum, but a longer time is required toobtain a sufficient amount of dehydration.

In a preferred aspect of this invention, the catalysts hereof areselected as those having a crystal framework density, in the dryhydrogen form, of not substantially below about 1.6 grams per cubiccentimeter. It has been found that zeolites which satisfy all three ofthese criteria are most desired because they tend to maximize theproduction of gasoline boiling range hydrocarbon products. Therefore,the preferred catalysts of this invention are those having a constraintindex as defined above of about 1 to about 12, a silica to alumina ratioof at least about 12 and a dried crystal density of not less than about1.6 grams per cubic centimeter. The dry density for known structures maybe calculated from the number of silicon plus aluminum atoms per 1000cubic Angstroms, as given, e.g., on page 19 of the article on ZeoliteStructure by W. M. Meir. This paper, the entire contents of which areincorporated herein by reference, is included in "Proceedings of theConference on Molecular Sieves, London, April 1967", published by theSociety of Chemical Industry, London, 1968. When the crystal structureis unknown, the crystal framework density may be determined by classicalpykometer techniques. For example, it may be determined by immersing thedry hydrogen form of the zeolite in an organic solvent which is notsorbed by the crystal. It is possible that the unusual sustainedactivity and stability of this class of zeolites is associated with itshigh crystal anionic framework density of not less than about 1.6 gramsper cubic centimeter. This high density of course must be associatedwith a relatively small amount of free space within the crystal, whichmight be expected to result in more stable structures. This free space,however, is important as the locus of catalytic activity.

Following the completion of synthesizing the zeolite, it is essential,for the purposes of this invention, to reduce or eliminate thenucleation of the aluminosilicate while at the same time keeping thecrystal growth high. To produce the outer aluminum-free shell, it isalso essential that the reactive aluminum be removed from the reactionmixture.

It is therefore necessary to process the zeolite and to replace thecrystallization medium with an aluminum-free mixture to obtaincrystallization of SiO₂ on the surface of the zeolite, the SiO₂ havingthe same crystal structure as the zeolite. This can be accomplished by atotal replacement of the reaction mixture or by complexing from theoriginal reaction mixture any remaining aluminum ion with reagents suchas gluconic acid, tartaric acid, nitrilotriacetic acid or EDTA. Inaddition, the OH-- concentration must be increased and the organic ionreduced so that the new reaction mixture, exclusive of solid crystals,has the following composition, in terms of mole ratios of oxides:

                  TABLE 5                                                         ______________________________________                                                                    Particularly                                              Broad    Preferred  Preferred                                         ______________________________________                                         ##STR13##                                                                               0.01-0.10  0.01-.08   0.02-0.06                                     ##STR14##                                                                               300-5000   500-5000   700-5000                                      ##STR15##                                                                               20-500     50-300     60-250                                        ##STR16##                                                                               0.05-1     0.1-0.8    0.2-0.6                                       ##STR17##                                                                               .1-2       0.15-1.5   0.2-1                                        ______________________________________                                         *R is an organic ion.                                                    

These ranges apply to the contemplated zeolite ZSM-5, ZSM-11, ZSM-12,ZSM-21 and ZSM-35. Typical reaction conditions include heating the abovemixture at a temperature of from about 80° C. to about 200° C. for aperiod of time from about 4 hours to about 30 days. As in the case ofZSM-5 aluminosilicate synthesis, the digestion of the gel particles iscarried out until the crystalline SiO₂ forms completely on the outershell of the zeolite particles. The product crystals are then separated,as by cooling and filtering, and are water washed and dried at fromabout 80° C. to about 150° C.

Members of the present family of zeolites can have the original cationsassociated therewith replaced by a wide variety of other cationsaccording to techniques well known in the art. Typical replacing cationswould include hydrogen, ammonium and metal cations including mixtures ofthe same. Of the replacing metallic cations, particular preference isgiven to cations of metals such as rare earth metals, manganese andcalcium, as well as metals of Group II of the Periodic Table, e.g., zincand Group VIII of the Periodic Table, e.g., nickel.

Typical ion exchange techniques include contacting the members of thefamily of zeolites with a salt of the desired replacing cation orcations. Although a wide variety of salts can be employed, particularpreference is given to chlorides, nitrates and sulfates.

Representative ion exchange techniques are disclosed in a wide varietyof patents including U.S. Pat. Nos. 3,140,249; 3,140,251; and 3,140,253.

Following contact with the salt solution of the desired replacingcation, the zeolites are then preferably washed with water and dried ata temperature ranging from 150° F. to about 600° F. and thereaftercalcined in air or other inert gas at temperatures ranging from about500° F. to about 1500° F. for periods of time ranging from 1 to 48 hoursor more.

Regardless of the cations replacing the sodium in the synthesized formof the catalyst, the spatial arrangement of the aluminum, silicon andoxygen atoms which form the basic crystal lattices of any given zeoliteof this invention remains essentially unchanged by the describedreplacement of sodium or other alkali metal as determined by taking anX-ray powder diffraction pattern of the ion-exchanged material. Forexample, the X-ray diffraction pattern of several ion-exchanged ZSM-5zeolites reveal a pattern substantially the same as that set forth inTable 1, above.

The aluminosilicates prepared by the instant invention are formed in awide variety of particular sizes. Generally speaking, the particles canbe in the form of a powder, a granule, or a molded product, such asextrudate having particle size sufficient to pass through a 2 mesh(Tyler) screen and be retained on a 400 mesh (Tyler) screen. In caseswhere the catalyst is molded, such as by extrusion, the aluminosilicatecan be extruded before drying or dried or partially dried and thenextruded.

In the case of many catalysts, it is desired to incorporate the catalystof this invention with another material resistant to the temperaturesand other conditions employed in organic conversion processes. Suchmaterials include active and inactive materials and synthetic ornaturally occurring zeolites as well as inorganic materials such asclays, silica and/or metal oxides. The latter may be either naturallyoccurring or in the form of gelatinous precipitates or gels includingmixtures of silica and metal oxides. Use of a material in conjunctionwith the present catalyst tends to improve the conversion and/orselectivity of the catalyst in certain organic conversion processes.Inactive materials suitably serve as diluents to control the amount ofconversion in a given process so that products can be obtainedeconomically and in orderly manner without employing other means forcontrolling the rate of reaction. Normally, zeolite materials have beenincorporated into naturally occurring clays, e.g., bentonite and kaolin,to improve the crush strength of the catalyst under commercial operatingconditions. These materials, i.e., clays, oxides, etc. function asbinders for the catalyst. It is desirable to provide a catalyst havinggood crush strength, because in a petroleum refinery the catalyst isoften subjected to rough handling, which tends to break the catalystdown into powder-like materials which cause problems in processing.These clay binders have been employed for the purpose of improving thecrush strength of the catalyst.

Naturally occurring clays which can be composited with the catalystinclude the montmorillonite and kaoline family, which families includethe sub-bentonites, and the kaolins commonly known as DixieMcNamee-Georgia and Florida clays or others in which the main mineralconstituent is halloysite, kaolinite, dickite, nacrite, or anauxite.Such clays can be used in the raw state as originally mined or initiallysubjected to calcination, acid treatment or chemical modification.

In addition to the foregoing materials, the catalyst can be compositedwith a porous matrix material such as silica-alumina, silica-magnesia,silica-zirconia, silica-thoria, silica-beryllia, silica-titania as wellas ternary compositions such as silica-alumina-thoria,silica-alumina-zirconia, silica-alumina-magnesia andsilica-magnesia-zirconia. The matrix can be in the form of a cogel. Therelative proportions of the finely divided crystalline aluminosilicatecontaining the aluminum-free outer shell and inorganic oxide gel matrixvary widely with the crystalline aluminosilicate content ranging fromabout 1 to about 90 percent by weight and more usually, particularlywhen the composite is prepared in the form of beads in the range ofabout 2 to about 50 percent by weight of the composite.

Employing the catalyst of this invention containing a hydrogenationcomponent, heavy petroleum residual stocks, cycle stocks, and otherhydrocrackable charge stocks can be hydrocracked at temperatures between400° F. and 850° F. using molar ratios of hydrogen to hydrocarbon chargein the range between 2 and 80. The pressure employed will vary between10 and 2,500 psig and the liquid hourly space velocity between 0.1 and10.

Employing the catalyst of this invention for catalytic cracking,hydrocarbon cracking stocks can be cracked at a liquid hourly spacevelocity between about 0.5 and 50, a temperature between about 550° F.and 1300° F., a pressure between about atmospheric and a hundredatmospheres.

Employing a catalytically active form of a member of zeolites of thisinvention containing a hydrogenation component, reforming stocks can bereformed employing a temperature between 700° F. and 1000° F. Thepressure can be between 100 and 1000 psig, but is preferably between 200and 700 psig. The liquid hourly space velocity is generally between 0.1and 10, preferably between 0.5 and 4 and the hydrogen to hydrocarbonmole ratio is generally between 1 and 20 preferably between 4 and 12.

The catalyst can also be used for hydroisomerization of normalparaffins, when provided with a hydrogenation component, e.g., platinum.Hydroisomerization is carried out at a temperature between 200° and 700°F., preferably 300° to 550° F., with a liquid hourly space velocitybetween 0.1 and 2, preferably between 0.25 and 0.50 employing hydrogensuch that the hydrogen to hydrocarbon mole ratio is between 1:1 and 5:1.Additionally, the catalyst can be used for olefin isomerizationemploying temperatures between 30° F. and 500° F.

Other reactions which can be accomplished employing the catalyst of thisinvention containing a metal, e.g., platinum, includinghydrogenation-dehydrogenation reactions and desulfurization reactions.In order to more fully illustrate the nature of the invention and themanner of practicing the same, the following examples are presented.

EXAMPLE 1

This example shows that crystal growth, without nucleation can beeffected at low TPA/SiO₂ ratios and that zeolites having the ZSM-5structure can be prepared from reaction mixtures essentially free ofaluminum.

To a stirred mixture of 63.3 g of Q-brand sodium silicate in 79.2 g ofwater at 100° C. was added a solution of 12.3 g of tetrapropylammonium(TPA) bromide, 8.7 g of NaBr, and 4.8 g of H₂ SO₄ in 120 g of water. Thereaction mixture had the following molar composition:

TPA/SiO₂ =0.15

SiO₂ /Al₂ O₃ =1400

H₂ O/OH=220

OH/SiO₂ =0.20

M₂ O/SiO₂ =0.48

Nucleation of ZSM-5 occurred after 7 days (stirred, 100° C.). After 10days a sample of the solid phase was essentially 100 percent crystallineZSM-5; it had a crystal size of 0.2-0.6μ.

To the reaction mixture was then added 100 g of water, followed by asolution of 126.6 g of Q-brand in 158 g of water and finally a solutionof 13.4 g of NaBr and 9.6 g of H₂ SO₄ in 140 g of water. A new gelformed which was stirred for 4 additional days. In this manner the TPAlevel of the mixture was reduced to give a composition:

TPA/SiO₂ =0.05

SiO₂ /Al₂ O₃ =1400

H₂ O/OH=220

OH/SiO₂ =0.27

M₂ O/SiO₂ =0.54

After these 4 days the mixture was filtered and washed free ofextraneous salts with water. The solid was ZSM-5 in essentially 100percent crystallinity. Scanning electron micrographs showed that thecrystals had now grown to about 1μ in diameter. There was no evidence ofnew, small crystals (≦0.2μ).

EXAMPLE 2

This example shows that a TPA/SiO₂ ratio >0 is necessary forsatisfactory growth.

To a stirred mixture of 74.3 g of Q-brand sodium silicate in 80 g of H₂O at 98° C. was added a solution of 7.4 g of H₂ SO₄, 3.8 g of TPABr, and0.8 sodium tartrate in 159 g of H₂ O. After thorough mixing, 7.7 g ofZSM-5 crystals were added (1-5μ, 0.76 percent N, 0.95 percent Na, SiO₂/Al₂ O₃ =73.9). The reaction mixture, exclusive of the seed crystal, hada composition as follows:

SiO₂ /Al₂ O₃ >1000

H₂ O/OH=450 p0 OH/SiO₂ =0.10

M₂ O/SiO₂ =0.29

R₂ O/M₂ O=0.07

TPA/SiO₂ =0.04

Stirring was continued for 5 days where upon the mixture was filtered toyield 19 g of ˜100 percent crystalline ZSM-5.

When, in a similar experiment, the TPABr was replaced by an equimolaramount of NaBr, the product after 5 days was only about 25 percentcrystalline, a crystallinity attributed to the initial seeds.

EXAMPLE 3

This example shows that complexing ligands can effectively removealuminum from crystallizing gels.

A reaction mixture having the following composition was placed in asteam box to crystallize:

SiO₂ /Al₂ O₃ =90

H₂ O/OH=450

OH/SiO₂ =0.10

M₂ O/SiO₂ =0.48

R₂ O/M₂ O=0.16

Such a mixture would normally yield a ZSM-5 product of SiO₂ /Al₂ O₃ ≅66in 17 days.

After 11 days a sample was taken, filtered, and the solid was analyzedby X-ray diffraction. It was 50 percent crystalline ZSM-5. To theremaining mixture was then added a solution of 0.7 g sodium gluconate in20 cc of water, a molar amount of gluconate equal to all the aluminumoriginally added. After 12 days the solid phase was 95 percentcrystalline ZSM-5 and had a SiO₂ /Al₂ O₃ of 131.

EXAMPLES 4-20

In these examples are summarized results for a range of reaction mixturecompositions and crystallization conditions.

Pre-formed, purified crystals of ZSM-5 of SiO₂ /Al₂ O₃ =72 were used inorder to separate the processes of nucleation and of growth. The SiO₂/Al₂ O₃ in these ranged from 67.9 to 79.7. All gels were prepared fromQ-brand sodium silicate (28.5% SiO₂, 7.75% Na₂ O). In a typicalpreparation a solution of 48.3 g of Q-brand and 52.0 g of water wasadded to a polypropylene bottle immersed in an oil bath at about 100° C.Attached to the bottle was a reflux condenser and a teflon stirringblade and shaft. A solution of 2.5 g of H₂ SO₄, 1.2 g of TPABr(tetrapropylammonium bromide) and 0.5 g of Na-tartrate in 103 g of waterwas added with stirring, followed by 5 g of powdered crystalline ZSM-5aluminosilicate zeolite. The reaction mixture, exclusive of the ZSM-5zeolite added, had the following mole ratios:

SiO₂ /Al₂ O₃ =>1000

H₂ O/OH⁻ =150

OH/SiO₂ =0.30

M₂ O/SiO₂ =0.28

where M is sodium and tetrapropylammonium cations.

Samples were periodically removed by suction, were filtered, washed,dried and analyzed by X-ray for crystallinity. After 8 days, the mixturewas 100% crystalline. It was filtered, boiled with water to removeextraneous salts, was filtered, dried and analyzed.

In these examples (4-20) there are three criteria for successfulcrystallization, namely, an increased product weight from that of theoriginal seeds, substantial retention of crystallinity, and an increasein SiO₂ /Al₂ O₃ ratio. None of the attempts without added TPA met thesecriteria, while those with TPA were all successful. The following Table6 lists the results.

                                      TABLE 6                                     __________________________________________________________________________    GROWTH OF ZSM-5 SEED CRYSTALS IN SiO.sub.2 - ONLY REACTION MEDIA*                                      Time   Product                                       Example                                                                            H.sub.2 O/OH                                                                       OH/SiO.sub.2                                                                       M.sub.2 O/SiO.sub.2                                                                 R/SiO.sub.2                                                                       Days                                                                             Seeds                                                                             Weight                                                                            Crystallinity                                                                        SiO.sub.2 /Al.sub.2 O.sub.3        __________________________________________________________________________    4    450  0.10 0.28  0.02                                                                              10   5 g                                                                              13 g                                                                               70%  193                                5    450  0.10 0.28  0   10 5   31   30    211                                6    450  0.30 0.28  0.02                                                                              5  5    9  100    148                                7    450  0.30 0.28  0   10 5   12   40    120                                8    150  0.30 0.28  0.02                                                                              8  5    9  100    151                                9    150  0.30 0.28  0.02                                                                              7  20  22  100    102                                10   150  0.30 0.28  0   14 5   --  --     --                                 11   100  0.60 0.32  0.02                                                                              13 5    7  100    90                                 12   100  0.60 0.31  0   13 5    5  100    69                                 13   150  0.30 0.28  0.02                                                                              3  10  22   70    153                                14   150  0.30 0.28  0   3  10  20   50    134                                15   450  0.10 0.28  0.02                                                                              4  10  34  100    217                                16   450  0.10 0.27  0   15 10  35   25    202                                17   150  0.30 0.28  0.02                                                                              7  20  21  100    98                                 18   150  0.30 0.28  0   18 20  --   70    89                                 19   150  0.30 0.28  0.02                                                                              6  20  25  100    94                                 20   150  0.30 0.28  0   14 20  --   70    74                                 __________________________________________________________________________     *All stirred at 200-300 rpm                                                   Examples 4 to 12 were as synthesized, large crystal seeds, 100° C.     Examples 13 and 14 were as synthesized, large crystal seeds, 160°      C.                                                                            Examples 15 to 18 were calcined, NH.sub.4 -form, large crystals,              100° C.                                                                Examples 19 and 20 were as synthesized, microcrystalline seeds,               100° C.                                                           

EXAMPLES 21 AND 22

These examples illustrate procedures for synthesizing layered ZSM-5crystallites without separation and purification of thealuminum-containing intermediate product. The experiments are detailedin Table 7.

Both examples were preceded by nucleation for 7 days in a reactionmixture as specified in the Table. This reaction mixture produced aZSM-5 product having a SiO₂ /Al₂ O₃ ratio=72 and thereby removed, inaddition to aluminum, 0.238 moles of SiO₂ together with about 0.005moles of TPA and of Na cations. At this point, additional reactants wereadded but with no further TPA. In this manner, the effective TPA/SiO₂ratio of the reaction mixture was reduced from 0.15 to 0.05. Followingan additional 7 days of crystallization, the experiment designatedExamples 21 and 22 were conducted to illustrate the calculations and theprocedures. In each case, reaction mixture compositions were calculatedafter subtracting that material already crystallized.

                                      TABLE 7                                     __________________________________________________________________________    MULTI-STEP CRYSTALLIZATION OF LAYERED ZSM-5 STIRRED, 100° C.           Moles in Reaction Mixture        Ratios Exclusive of Zeolite                  Operation                                                                           SiO.sub.2                                                                         Al.sub.2 O.sub.3                                                                   TPA.sub.2 O                                                                        Na.sub.2 O                                                                          OH  H.sub.2 O                                                                        SiO.sub.2 /Al.sub.2 O.sub.3                                                         H.sub.2 O/OH                                                                       OH/SiO.sub.2                                                                        M.sub.2 O/SiO.sub.2                                                                  TPA/SiO.sub.2        __________________________________________________________________________    Nucleation                                                                          0.301                                                                             0.0033                                                                             0.023                                                                              0.079 0.030                                                                             13.5                                                                             90    450  0.10  0.34   0.15                  7 days                                                                       Less solid                                                                          -0.238                                                                            -0.0033                                                                            -0.005                                                                             -0.005                                                                              -0.007                                                                            --                                              Add gel                                                                             0.601                                                                             0.0066                                                                             0    0.158 0.036                                                                             16.4                                            Growth                                                                              0.664                                                                             0.0066                                                                             0.018                                                                              0.232 0.059                                                                             29.9                                                                             101   500  0.09  0.38   0.05                  7 days                                                                       Example 21                                                                    Less solid                                                                          -0.475                                                                            -0.0066                                                                            -0.009                                                                             -0.009                                                                              -0.013                                                                            --                                              Add gel                                                                             0.301                                                                             0    0    0.079 0   2.3                                             Shell 0.490                                                                             0    0.009                                                                              0.302 0.046                                                                             32.2                                                                             >1000 700  0.09  0.63    0.035                3 days                                                                       Product                                                                             [1.04                                                                             0.011                                                                              0.021                                                                              0.057].sup.a (95)  --   --    --     --                   ZSM-5                                                                         Example 22                                                                    Less solid                                                                          -0.475                                                                            -0.0066                                                                            -0.009                                                                             -0.009                                                                              -0.013                                                                            --                                              Add gel                                                                             0.601                                                                             0    0    0.158 0.191                                                                             4.6                                             Shell 0.790                                                                             0    0.009                                                                              0.381 0.237                                                                             34.5                                                                             >1000 150  0.30  0.49   0.02                  7 days                                                                       Product                                                                             [0.97                                                                             0.010                                                                              0.018                                                                              0.022].sup.a (96)  --   --    --     --                   ZSM-5                                                                         __________________________________________________________________________     .sup.a Material balance obtained on product zeolite                      

EXAMPLE 23

In Example 23, sorptive properties of layered ZSM-5 products were testedas a measure of channel and crystal integrity.

That the product crystals possess an intact and accessible pore system,if somewhat modified, was demonstrated by measuring sorptive capacitieson calcined as-synthesized products. Sorptive capacities for n-hexane,20 mm, 25° C., were 11.3%, 10.9% and 11.1% for Examples 9, 19 and 21,respectively, vs. 11.1% for untreated ZSM-5's.

EXAMPLE 24

This example shows that ZSM-5 samples treated in accordance with theseprocedures are useful and are selective in the conversion ofhydrocarbons.

The product of Example 9 was calcined in flowing nitrogen to 550° C.,cooled, and exchanged with 2 M NH₄ NO₃ to yield the NH₄ -form of thezeolite. After calcination to 550° C. in air, the active zeolitecatalyst (sized to 60/80 mesh) was contacted with a five componenthydrocarbon feed as follows:

Feed=Equal weight mixture of n-hexane, 3-methylpentane,2,3-dimethylbutane, benzene and toluene

WHSV=3.1

Temperature=316° C.

H₂ /Hydrocarbon=3.6

Pressure=200 psig

Conversion was measured at 5 and at 25 hours on stream and averaged:hexane=78%, 3-methylpentane=15%, 2,3-dimethylbutane=1%, benzene=7% andtoluene=4%. During this test reaction, alkylation and rearrangement ofaromatics occurs. In this example, over 10% of the paraffins crackedwere incorporated into the liquid product as alkyl groups on the feedbenzene and toluene. Xylenes, produced in the rearrangement of alkylaromatics, were found to be unusually high in para-isomers. The molarratio of para/meta-xylene was 1.1 as compared with a ratio of 0.6 overuntreated large crystal ZSM-5 catalysts and a thermodynamically expectedratio of 0.45.

I claim:
 1. A crystalline aluminosilicate zeolite having analuminum-free outer shell of SiO₂, said outer shell having the samecrystal structure as said zeolite.
 2. The zeolite of claim 1 having aconstraint index of from 1 to
 12. 3. The crystalline aluminosilicatezeolite of claim 1 having the crystal structure of ZSM-5, ZSM-11, ZSM-12or ZSM-35.
 4. The zeolite of claim 3 having the crystal structure ofZSM-5.
 5. The zeolite of claim 3 having the crystal structure of ZSM-11.6. The zeolite of claim 3 having the crystal structure of ZSM-12.
 7. Thezeolite of claim 3 having the crystal structure of ZSM-35.